1. Field of the Invention
This invention relates to apparatus for joining overlapped metal sheets and, more particularly, to an apparatus for continuously deforming the sheets so as to resist separation of the sheets when subjected to shear and peeling forces. The invention is also directed to a method of joining the sheets, as with the inventive apparatus.
2. Background Art
It is well known to join lapped sheets of metal by deformation thereof, thereby obviating the need to weld or use separate fasteners, such as rivets, to effect joining. This process is commonly referred to in the trade as "metal stitching".
Metal stitching is commonly carried out on a reciprocating press. An exemplary system is currently offered by BTM Corporation of Marysville, Mich. The system employs a punch and die pair which permit a "stitch" to be formed with a single stroke of the punch.
The die is constructed to assume two different states. In the first state, the die defines a blind receptacle into which the sheet material can be drawn as the punch advances in a first line. At a predetermined point in the punch stroke, after the underlying sheet is lanced, the die spreads under pressure transversely to the line of the punch stroke to allow the drawn part of the overlying sheet to expand transversely to the line of the punch stroke. This causes the undeformed part of the underlying sheet to be sandwiched between the undeformed part of the overlying sheet and the dram part of the overlying sheet that is expanded transversely to the first line to under the second sheet.
While this type of system creates an effective "stitch", such systems are impractical for high volume "stitching". The operator thereof is required to serially align the sheets, extend the punch, retract the punch, and thereafter shift the sheet metal stock to repeat the "stitching" operation on a downstream portion of the stock. Such a system inherently requires either a substantial amount of operator involvement or an automated system that performs the "stitching" operation in somewhat inefficient steps.
Further, the requirement of using a die with movable parts makes the die more prone to failure than a fixed configuration die. At the same time, the more intricate, reconfigurable die may be more expensive to manufacture than a fixed configuration die.